<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<body>
    <ul class="content1">
        <li>1</li>
        <li>2</li>
        <li>3</li>
    </ul>
    <ul class="content2">
        <li>a</li>
        <li>b</li>
        <li>c</li>
    </ul>
    <script>
       let  ul1 = document.getElementsByClassName('content1')[0]
    //    let  ul2 = document.querySelectorAll('.content2 li')
        let ul2=document.getElementsByClassName('content2')[0]
        // ul2.forEach(element => ul1.insertBefore(element.cloneNode(true), ul1.firstElementChild))
        for (let i = 0, len = ul2.children.length; i < len; i++) {
            
            ul1.insertBefore(ul2.children[len- i-1], ul1.firstElementChild)

        }




    </script>
</body>

</html>